1. Field of the Invention
The present invention relates to focus detection devices, and particularly, to an automatic focus detection device using a base line distance measuring system.
2. Description of the Prior Art
Various kinds of automatic focus detection devices of the so-called active type, using a light measuring light source and a light projection lens, have been proposed. These scan an object with a light beam from the light source and detect a focus condition by means of a signal from a light sensing element when the light, reflected by the object and passing through the light receiving lens, forms an image on the light sensing element.
To scan with this kind of device, both the light projection image forming lens and the light source lens are rotated, only the lens is moved, or only the light source is moved.
In such devices, when the positions of the image forming lens and the light source are adjusted so that the image is formed correctly on an object at one distance, the image of the light source is not formed correctly on other objects spaced from the first object, and hence, is not sharp. Consequently, when the light beam is reflected and sensed by the light sensing element, it is not possible to detect the peak value correctly. This adversely affects the accuracy of the distance measurement. In order to overcome this disadvantage, U.S. Pat. No. 3,435,744 discloses varying the distance between the lens, the light source, and the light sensing element in cooperation with the focus detection device. Although this type of device can produce a sharp image on the object, and the light sensing element so as to improve the accuracy of the distance measurement, the construction is complicated. Therefore, manufacturing costs are unavoidably high.